The present invention relates to a process for preparing nitroaromatic nitriles by the nitration of aromatic carbocyclic nitriles with a mixture comprising nitric acid and sulfuric acid at a temperature of at least 35.degree.C, preferably as high as 40.degree.C, and an acids-to-nitrile mol ratio of at least 3.5:1.
The use of nitric acid and mixtures of nitric acid and sulfuric acid to nitrate aromatic carbocyclic nitriles heretofore has been difficult or impossible to accomplish because the nitrating acids tend to hydrolyze the nitrile moiety. Various suggestions have been offered to overcome the hydrolization problem. The usual solution has been to refrigerate the reaction vessel so as to maintain a reaction temperature below about 0.degree.C. As early as 1928, Baker et al., J. Chem. Soc. 1928, 426, reported that nitric acid could be utilized to nitrate benzonitrile at quantitative yields at 0.degree.C. Since that time, attempts have been made to effect nitration of aromatic carbocyclic nitriles without the need to employ costly equipment and inefficient methods of synthesis.
U.S. Pat. No. 3,162,675, granted Dec. 22, 1964, discloses the use of nitronium salts as high-temperature nitrating agents. Nitronium fluoroborate, NO.sub.2 BF.sub.4, is specifically suggested as a convenient and available nitrating agent.
Hammond et al., J. Amer. Chem. Soc. 81, 1959, pp. 1184-7, describe the use of perchloric acid and nitric cid mixtures as nitrating agents which may be employed at temperatures as high as 43.degree.C.
Hetherington and Robinson, Chem. Soc. J. (London), 1954. Pt. 3, pp. 3512-14, describe the use of nitryl fluoride as a nitrating agent which may be employed with aromatic substrates.
Olah et al, Chem. Soc. J. (London), 1956, Pt. 4, pp. 4257-8, describe the use of nitronium tetrafluoroborate and other nitronium salts to effect nitration of aromatics at temperatures as high as 130.degree.C.